The present invention generally relates to polymer compositions having high thermal conductivity and high density. The polymer composition can be used to make a thermally-conductive housing for electrical motors having a rotor and stator assembly.
High speed motors are well known and used in many different applications. For example, high speed motors having a rotor and stator assembly are used in home appliances, industrial equipment, and computer disc drives. The components of the motor must be kept clean from contaminating particles and other foreign matter that can interfere with their operation. One method for protecting such motors involves encapsulating or overmolding the motor with a plastic composition. For example, a plastic resin such as a polycarbonate, polystyrene, styrene copolymer, polyolefin, acrylate, acrylic, polyvinyl chloride, polyester, or polyamide resin can be used to encapsulate the motor. Such conventional plastic compositions are generally effective in protecting the components of the motor from hazardous environmental conditions such as exposure to corrosive fluids, contamination from dirt and dust particles, and other materials. Also, such compositions are good electrical insulators. Further, plastic compositions can be used to improve the mechanical integrity and other properties of the motor assembly. However, conventional plastic compositions have some drawbacks.
Particularly, the motor can generate a substantial amount of heat that must be removed in order for the device to function properly. If the heat is not efficiently dissipated, the motor can overheat, and the device can break-down. Conventional plastic compositions generally are good thermal insulators. These compositions are inefficient for removing heat and cooling the motor.
To address this problem, plastic compositions having improved thermally conductive properties have been developed. Neal, U.S. Pat. No. 6,362,554 discloses a method of encapsulating a high speed spindle motor that includes a core and a stator having multiple conductors. These conductors create magnetic fields as they conduct electrical current. A thermally-conductive body encapsulates the stator. The '554 Patent discloses that a thermally-conductive, but non-electrically-conductive, plastic composition including ceramic filler particles can be used to form the encapsulating body. According to the '554 Patent, a preferred form of plastic is polyphenyl sulfide, and the amount of glass, “Kevlar”, carbon or other fibers in the material can be varied. The '554 Patent also discloses that the amount of ceramic material in the material can be varied.
Although such thermally-conductive, plastic compositions can be somewhat effective in transferring heat away from the motor assembly, there is a need for an improved composition that can help with motor balancing problems. This problem can occur in high speed motors having a rotor and stator assembly. The stator includes a magnetically-inducible core having poles extending therefrom. Copper wire windings are wound around the poles and serve as conductors. The copper wire windings create magnetic fields in the core as the windings conduct electrical current. The motor further includes a rotor shaft which is connected to a hub and surrounded by bearings. The magnetic field interacts with the rotor shaft and hub causing these pieces to rotate.
One problem with the winding of the copper wire around the poles is that the winding pattern can be non-uniform and this leads to balancing problems. The non-uniform winding problem is particularly pronounced in the outermost wraps (traverses) of the wound wire. This uneven distribution of the windings can cause vibrations as the shaft and hub rotate. As a result, manufacturers often add or remove material to balance the motor assembly and dampen the vibration. This time-consuming machining method of balancing the assembly can be costly.
It would be desirable to have a thermally-conductive, plastic composition that could be used to encapsulate the stator assembly uniformly so that these machining and balancing steps could be eliminated. The composition should have good heat-conduction properties so that it can dissipate heat from the motor assembly quickly and efficiently. The composition should also be relatively dense and capable of uniformly encapsulating and balancing the motor.
The present invention provides such a thermally-conductive, plastic composition. These and other objects, features, and advantages of this invention are evident from the following description and attached figures.